The present invention relates to web supporting and drying apparatus. In drying a moving web of material, such as paper, film or other sheet material, it is often desirable that the web be contactlessly supported during the drying operation, in order to avoid damage to the web itself or to any ink or coating on the web surface. A conventional arrangement for contactlessly supporting and drying a moving web includes upper and lower sets of air bars extending along a substantially horizontal stretch of the web. Heated air issuing from the air bars floatingly supports the web and expedites web drying. The air bar array is typically inside a dryer housing which can be maintained at a slightly sub-atmospheric pressure by an exhaust blower that draws off the volatiles emanating from the web as a result of the drying of the ink thereon, for example. The exhausted gases can then be treated to oxidize any volatile components, and the resulting clean gases can then be released to atmosphere.
Temperatures sufficient to fully oxidize the volatiles (typically in the 1250.degree. F. to 1500.degree. F. (675.degree. C.-815.degree. C.) range) are not reached in dryers of this type. Nor is sufficient residence time or mixing provided to cleanly treat the volatiles, for example. Indeed, it is desirable to avoid, or mitigate to the greatest extent possible, the partial oxidation and cracking of the volatiles, as partially oxidized and cracked compounds are often more deleterious than volatile material which has undergone little or no decomposition. The former may result from incomplete combustion due to insufficient oxygen, arrested combustion or insufficient temperature and length of time for the reaction to be completed, resulting in the generation of soot, carbon black, aldehydes, organic acids and carbon monoxide. The condensation and formation of the solids of these unwanted compounds on the internal surfaces of the drying apparatus are undesirable, as high accumulations may contaminate the web and product, may eventually adversely affect the operation of the dryer, and may present a fire hazard.
Additionally, it is desirable to provide make-up air to the dryer in such a way that internal surfaces are not unduly cooled, thus causing sites for the formation of condensation and solids of incomplete combustion.
It is therefore an object of the present invention to mitigate condensation and sapping of solvent and solvent-based by-products in an industrial dryer.
It is a further object of the present invention to provide for more thorough mixing of dryer atmosphere in order to maintain even solvent concentrations throughout the dryer enclosure.